In wireless communications systems, HARQ (Hybrid-Automatic-Repeat-Request) protocols are responsible for error control between transmitters and receivers.
One common existing approach is referred to as the “stop and wait” (SAW) approach involves the insertion of the re-transmit packet into the normal packet transmission flow, with the sequence of scheduled normal packet transmission stalled to allow re-transmission of the erroneous packet. One problem with this approach is that the re-transmit packets take away from the bandwidth available for new transmissions, and the re-transmit packets introduce delay into all subsequent re-transmissions. This approach is shown in FIG. 1A where the packets numbered “1” to “4” to be transmitted are generally indicated by 20, and the stream of actual packet transmissions is indicated generally by 22, and the acknowledgement sequence is indicated at 24. After a NAK (negative acknowledgement) indicating failed transmission is received in respect of packet “1”, the packet is re-transmitted after packet “2”, with the remaining packets being forced to wait. When an ACK acknowledgement) is received indicating successful transmission of packet “2”, packet “3” is then transmitted and so on.
In another existing approach shown in FIG. 1B, rather than re-transmitting a packet identical to the original packet, the re-transmitted packets contain the entire systematic content of the original packet but contain different versions of the redundancies. Thus, in the packet flow 30 of FIG. 1B, packet “1(1)” is a re-transmitted version of packet “1” with a first revised set of redundancies, and the packet “1(2)” is a re-transmitted version of packet “1” with a second revised set of redundancies. These re-transmitted packets are shown inserted directly within the transmission flow, so this is a stop and wait scheme. Alternatively, the re-transmission flow can be transmitted on a separate physical channel as shown in FIG. 1C.
Another approach which has been employed is referred to as AAIR (asynchronous adaptive incremental redundancy). In this approach, packets are divided up into three segments, the first containing a systematic component and some redundancy, and the second and third segments only containing redundancy. In the event the initial packet transmission is in error, the first segment is re-transmitted. If the packet still cannot be recovered, the second segment is re-transmitted. Finally, if the packet still cannot be recovered, the third segment is re-transmitted. The original transmission plus the re-transmitted segments are combined using Chase and incremental redundancy (IR) combining.
U.S. patent application Ser. No. 10/074,701 entitled “Partial Puncture Re-Transmission” filed Feb. 13, 2002 and commonly assigned with this application hereby incorporated by reference in its entirety, teaches another incremental redundancy approach referred to as the Non-complete puncture (NCP) approach in which for re-transmission, re-transmit packets are formed with different versions of redundancy as in the example of FIG. 1B. Each re-transmit packet is divided into a large number of segments or sub-packets, for example six per packet. The bits of these sub-packets, one sub-packet at a time, are inserted by puncturing them into the regular transmission stream, thereby having no effect on the regular transmission schedule. One issue with this approach is that it is possible that a large number of re-transmissions will be required before sufficient redundancy is re-transmitted to allow successful reception. For example, compared to simply re-transmitting the packet three times, re-transmission using ⅙th sub-packets may take up to 18 time slots and this delay may be too long. Another issue with this approach is that at the receiver, the entire “soft packet” received so far needs to be stored, and this can lead to significant buffer requirements since many packets can be transmitted and many re-transmissions may be occurring simultaneously. This approach does offer the benefit of time diversity. This NCP approach is depicted in FIG. 2 where the regular packet flow is indicated generally by 40, and the ACK/NAK stream is indicated at 42. For packet “1” which was received in error, a re-transmit packet 44 is generated. This is referred to as “1(1)” because subsequent re-transmit packets may be employed which would contain different versions of redundancy. Also shown is a re-transmit packet 46 for packet “3” which was also received in error. Each of these re-transmit packets is in turn is divided into respective sets of sub-packets, the illustrated example showing six sub-packets per re-transmit packet. These re-transmit sub-packets are then inserted by puncturing, one sub-packet at a time in each slot of the regular transmission. Thus, the first sub-packet of the first re-transmit packet 44 is inserted into by puncturing packet “3”, the second sub-packet of the first re-transmit packet 44 is inserted by puncturing into packet “4”, and so on.